A Syrian military helicopter fired rockets at a pro-rebel region of eastern Lebanon in the early hours of Thursday, a security source told AFP.
"A military helicopter violated Lebanese airspace and fired four rockets at 01:30 am (2230GMT) in the Arsal area, two of which exploded, causing damage," the source said on condition of anonymity.
The attack did not cause any injuries.
Arsal is a Sunni neighbourhood in eastern Lebanon that is broadly sympathetic to the Syrian uprising against President Bashar al-Assad, and has become a transit point for Syrian refugees, as well as rebels and their weapons.
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The area has been targeted on multiple occasions by Syrian regime forces, including in a June 12 attack that hit the centre of Arsal.
That raid prompted a rare warning from the Lebanese army, which threatened to respond if the attacks continued.
The Syrian conflict has increasingly spilled over into Lebanon, and has raised sectarian tensions in the fragile country, which experienced a devastating 1975-1990 civil war.
Lebanon's powerful Shiite Hezbollah movement has sent fighters to battle alongside the regime, and many Lebanese Sunnis sympathise with the Sunni-dominated Syrian opposition.